Exploding Engine Could Reduce Fuel Consumption

Exploding Engine Could Reduce Fuel Consumption

Why It Matters

Fossil fuels will be needed for some time, because they are less expensive and higher in energy density than alternatives. Advanced engine designs offer some of the best options for using them more efficiently.

A new kind of engine under development, called a detonation engine, could save the military hundreds of millions of dollars in fuel costs every year. The technology, which military researchers are working on together with scientists at GE and other companies, could reduce fuel consumption at power plants, in ships, and on airplanes by as much as 25 percent. The Navy alone estimates that retrofitting its ships with the technology would reduce annual fuel costs by $300 to $400 million.

It could be over a decade before such engines are put to practical use. But DARPA, having finished detailed plans, is now in the middle of a $62 million program aimed at building the first full-scale demonstration of one version of the technology. (GE is involved in the project: see “GE’s Risky Research.”) Meanwhile, Navy researchers are using sophisticated simulations to advance a version of the concept that could make it far more practical.

Detonation engines would replace jet engines in airplanes and the gas turbines that run power plants and Navy ships. A set of rotating blades at the front of those engines compresses air, which is then mixed with fuel and combusted in a steady flame. That produces hot gases that do the work an engine is designed to do, whether it’s turning a propeller, propelling a jet, or spinning a generator to produce electricity.

Improving the efficiency of conventional jet engines has involved finding ways to increase air compression. But the cost and complexity of that approach is making it harder to realize improvements. Detonation engines offer another way to achieve high pressures. In a detonation engine, fuel combustion generates a shock wave that raises pressures to levels 10 times those inside a conventional engine. “It’s like an explosion or a bomb,” says Kazhikathra Kailasanath, a researcher at the Naval Research Laboratory in Washington, DC. “If you burn something in an open flame, the pressure stays the same as the surrounding pressure. The big difference with a detonation engine is going from that to a confined type of combustion, where the pressure goes up and the combustion occurs more rapidly.”

The most highly developed form of detonation engine, which has been in the works for many years, is the pulse detonation engine, the type GE is developing. Whereas combustion occurs continuously in a conventional jet engine, pulse detonation involves setting off a series of detonations—say, 60 to 100 per minute.

The Naval Research Laboratory has another idea. It involves the use of a specially designed doughnut-like combustion chamber. One explosion is set off with a spark in one part of the chamber. As the shock wave propagates out from that explosion, the researchers keep it going by feeding in a precise mixture of fuel and air ahead of it. A handful of research groups have tested small versions of the engine that burn hydrogen. And the Navy researchers recently published a paper that shows the idea can work with hydrocarbon fuels like the ones that would be used in a ship, at least in detailed computer simulations. An advantage of this approach is that it produces a constant stream of hot gases, which more closely resembles what’s seen in a conventional jet engine. It’s also simpler, in that there’s no need to engineer a system to create detonations at a high rate.

Kailasanath says that while people had dreamed of making detonation engines for decades, it’s only the advent of advanced computer simulations that is making it possible to understand the fast reactions involved. Many challenges still remain, especiallly building engines that are strong enough to withstand the detonations. That’s easier to do for stationary application like power plants, where the weight of the engine isn’t much of an issue. But detonation engines for airplanes might require new materials. They also require careful engineering, says Narendra Joshi, advanced technology leader for propulsion technologies at GE. “The detonation is like a hammer blow,” he says. “You have to be careful where that hammer blow goes.”